//响应头中间件
module.exports = async (ctx,next)=>{
    const contentType = 'application/json; charset=utf-8';
    ctx.set('Content-Type',contentType);
    ctx.set("Access-Control-Allow-Origin", "*");
    ctx.set("Access-Control-Allow-Methods", "OPTIONS, GET, PUT, POST, DELETE");
    await next();
}